Supramolecular Structures of Zinc (II) (8-Quinolinolato) Chelates^†

We investigate the oligomeric purity and stability of zinc (8-quinolinolato) (Znq₂) and its methylated derivatives (nMeq₂Zn, n = 2, 4, 5) through a combination of theoretical modeling of oligomerization energetics leading to supramolecular structures and experimental size-exclusion chromatography studies. Gas- and solution-phase (CHCl₃) formation energies for dimeric, trimeric, and tetrameric species are reported. Favorable gas-phase thermodynamics were calculated and found to favor tetrameric structures for all Znq₂ chelates (∼−50 kcal/mol for monomer dimerization to ∼−35 kcal/mol for dimer dimerization), with the exception of 2Meq₂Zn, which gave lower formation energies by 30−45% due to steric hindrance. Solvation model computations indicate that these energies are reduced by ∼10−25% with the introduction of a dielectric medium. Computed structural parameters for the basic Zn−O core structure formed via bridging of phenolato oxygens do not change significantly as oligomer growth progresses. Size-exclusion chromatography experiments of crystalline and amorphous films (vapor deposited) dissolved in CHCl₃ or CHCl₃/DMSO mixtures showed that the dominant species for Znq₂, 4Meq₂Zn, and 5Meq₂Zn is tetrameric, but partial disassociation to monomers can occur in the presence of nucleophilic solvent. The sterically hindered 2Meq₂Zn was monomeric in all solvent systems. Implications for organic light-emitting devices using these materials are discussed.